digitalmars.D.learn - get parameters of a function
- Alex (22/22) Apr 28 2017 Hi all,
- Stanislav Blinov (20/25) Apr 28 2017 Rather, it is to be used on a concrete overload.
- Alex (30/46) Apr 28 2017 The problem is another one: say I have something like this:
- Stanislav Blinov (83/111) Apr 29 2017 Ah, that calls for something like a isCallableWith template. Pay
- Alex (6/88) Apr 29 2017 :)
Hi all, I have a question about the Parameters trait from https://dlang.org/phobos/std_traits.html#Parameters The following code does not compile. Why? import std.traits : Parameters; void main() { static assert(is(Parameters!S1 == Parameters!S2)); } struct S1 { auto opCall() {} auto opCall(int i){} } struct S2 { auto opCall(int i){} auto opCall() {} } So, of course, if I change the both opCall methods, e. g. in S1 it will. But I'm wondering, how to use the trait in this case? Is it mainly assumed to use it with functions without overloads?
Apr 28 2017
On Friday, 28 April 2017 at 20:43:50 UTC, Alex wrote:Hi all, I have a question about the Parameters trait from https://dlang.org/phobos/std_traits.html#Parameters The following code does not compile. Why? Is it mainly assumed to use it with functions without overloads?Rather, it is to be used on a concrete overload. If you want to check if two structs are callable with the same parameters, you'll need to iterate opCall overloads for both and look for a match, e.g. like this: void main() { foreach(o1; __traits(getOverloads, S1, "opCall")) { alias P1 = Parameters!o1; foreach(o2; __traits(getOverloads, S2, "opCall")) { alias P2 = Parameters!o2; static if (is(P1 == P2)) { pragma(msg, "Matching params: "~P1.stringof); } } } }
Apr 28 2017
On Saturday, 29 April 2017 at 05:58:35 UTC, Stanislav Blinov
wrote:
On Friday, 28 April 2017 at 20:43:50 UTC, Alex wrote:
void main()
{
foreach(o1; __traits(getOverloads, S1, "opCall"))
{
alias P1 = Parameters!o1;
foreach(o2; __traits(getOverloads, S2, "opCall"))
{
alias P2 = Parameters!o2;
static if (is(P1 == P2))
{
pragma(msg, "Matching params: "~P1.stringof);
}
}
}
}
The problem is another one: say I have something like this:
import std.traits;
struct A(alias T) if(isCallable!T)
{
auto opCall(U...)(U args) if(is(Parameters!T == U))
//if(__traits(compiles, T(args)))
{
return T(args);
}
}
void main()
{
S s;
A!s c;
assert(c(4) == 42);
}
struct S
{
auto opCall(){ return 42; }
auto opCall(int i){return 42; }
}
This doesn't work because of Parameters trait and the both
opCalls defined.
However, I see the point...
If I take the compiles-trait, it works as expected:
In this case the constrained is fulfilled, and with the second
opCall commented out - it isn't.
Thanks :)
Apr 28 2017
On Saturday, 29 April 2017 at 06:18:34 UTC, Alex wrote:
The problem is another one: say I have something like this:
import std.traits;
struct A(alias T) if(isCallable!T)
{
auto opCall(U...)(U args) if(is(Parameters!T == U))
//if(__traits(compiles, T(args)))
{
return T(args);
}
}
void main()
{
S s;
A!s c;
assert(c(4) == 42);
}
struct S
{
auto opCall(){ return 42; }
auto opCall(int i){return 42; }
}
This doesn't work because of Parameters trait and the both
opCalls defined.
However, I see the point...
If I take the compiles-trait, it works as expected:
In this case the constrained is fulfilled, and with the second
opCall commented out - it isn't.
Thanks :)
Ah, that calls for something like a isCallableWith template. Pay
extra care to how you pass parameters in that variadic opCall
template though. Doing it like in the code above will pass
everything by value, even though the original S.opCall might've
expected a reference. To avoid that, auto ref +
std.functional.forward can be used.
The following code illustrates the difference.
import std.traits;
import std.functional : forward;
/**
Evaluates true if `func` is callable with arguments `args`.
Can only be used as a function template constraint.
*/
enum isCallableWith(alias func, args...) =
is(typeof(func(forward!args)));
// Your original A
struct A(alias func) if (isCallable!func)
{
auto opCall(Args...)(Args args) if (__traits(compiles,
func(args)))
{
// all arguments are copied, any "ref/out" semantics are
lost
return func(args);
}
}
// Modified to forward arguments
struct B(alias func) if (isCallable!func)
{
// auto ref: preserve lvalues
auto opCall(Args...)(auto ref Args args) if
(isCallableWith!(func, args))
{
// forward: preserve lvalues
return func(forward!args);
}
}
struct S
{
auto opCall() { return 42; }
auto opCall(int i) { return 42; }
// Note: parameter passed by reference!
auto opCall(out float f) { f = 42.0f; }
}
void main()
{
S s;
{
A!s c;
int i = 4;
static assert(is(typeof(c(i))));
static assert(is(typeof(c(4))));
static assert(is(typeof(c())));
assert(c(i) == 42);
assert(c(4) == 42);
assert(c() == 42);
// Should pass but doesn't.
//static assert(!is(typeof(c(3.5f))));
float f;
static assert(is(typeof(c(f))));
c(f);
// Will assert:
assert(f == 42.0f);
}
{
B!s c;
int i = 4;
static assert(is(typeof(c(i))));
static assert(is(typeof(c(4))));
static assert(is(typeof(c())));
assert(c(i) == 42);
assert(c(4) == 42);
assert(c() == 42);
// Passes
static assert(!is(typeof(c(3.5f))));
float f;
static assert(is(typeof(c(f))));
c(f);
// Won't assert
assert(f == 42.0f);
}
}
Apr 29 2017
Alex <sascha.orlov gmail.com> On Saturday, 29 April 2017 at 08:15:06 UTC, Stanislav Blinov wrote:Ah, that calls for something like a isCallableWith template. Pay extra care to how you pass parameters in that variadic opCall template though. Doing it like in the code above will pass everything by value, even though the original S.opCall might've expected a reference. To avoid that, auto ref + std.functional.forward can be used.:)The following code illustrates the difference. import std.traits; import std.functional : forward; /** Evaluates true if `func` is callable with arguments `args`. Can only be used as a function template constraint. */ enum isCallableWith(alias func, args...) = is(typeof(func(forward!args)));// That's cool! Much shorter, as I thought.// Your original A struct A(alias func) if (isCallable!func) { auto opCall(Args...)(Args args) if (__traits(compiles, func(args))) { // all arguments are copied, any "ref/out" semantics are lost return func(args); } } // Modified to forward arguments struct B(alias func) if (isCallable!func) { // auto ref: preserve lvalues auto opCall(Args...)(auto ref Args args) if (isCallableWith!(func, args)) { // forward: preserve lvalues return func(forward!args); } } struct S { auto opCall() { return 42; } auto opCall(int i) { return 42; } // Note: parameter passed by reference! auto opCall(out float f) { f = 42.0f; } } void main() { S s; { A!s c; int i = 4; static assert(is(typeof(c(i)))); static assert(is(typeof(c(4)))); static assert(is(typeof(c()))); assert(c(i) == 42); assert(c(4) == 42); assert(c() == 42); // Should pass but doesn't. //static assert(!is(typeof(c(3.5f)))); float f; static assert(is(typeof(c(f)))); c(f); // Will assert: assert(f == 42.0f); } { B!s c; int i = 4; static assert(is(typeof(c(i)))); static assert(is(typeof(c(4)))); static assert(is(typeof(c()))); assert(c(i) == 42); assert(c(4) == 42); assert(c() == 42); // Passes static assert(!is(typeof(c(3.5f)))); float f; static assert(is(typeof(c(f)))); c(f); // Won't assert assert(f == 42.0f); } }Ha, that is exactly what I am after :) Thanks a lot!
Apr 29 2017